A group of students who start a course together on a set date, as against the usual case where everyone starts whenever they like.
Four dates, two meanings
Enrolment opens and closes says when people can sign up. The course starts and ends says when it actually runs.
The two pairs are independent: enrolment can open months before the start, and can stay open after the course has begun.
If somebody cannot enrol, look at the enrolment dates rather than the start date.
Capacity is optional
You can set a limit or leave it off. Without one, enrolment continues until the enrolment window closes.
A course can be both scheduled and self-paced
Having a cohort does not remove the ordinary case. One course can be sold self-paced and also run scheduled groups.